1. Field of the Invention
The present invention relates to a combination of a cylinder liner and a piston ring, more particularly, to a combination of a cylinder liner used for an internal-combustion engine for automobiles, lawn mowers, generators, etc. and a piston ring which slides on an inner circumference surface of such cylinder liner.
2. Related Art
In recent years, in order to improve fuel economy and kinematic performance, a reduction in weight of a whole automobile has been promoted, and, with such reduction in weight or a whole automobile, it is natural that the engine has also become a target of the reduction in weight.
In order to try to reduce the weight of an engine, it has been developed to use light alloy typified by magnesium alloy or aluminum alloy for a cylinder liner and a cylinder block which is cast so as to envelope the cylinder liner. As an illustrative embodiment of these cylinder liners and cylinder blocks, for example, Patent Literatures 1 to 5 can be mentioned.
In Patent Literatures 1 to 3, a cylinder liner formed of aluminum alloy using fiber-reinforced materials and a cylinder block made of aluminum alloy which was cast so as to envelope the cylinder liner are disclosed. In addition, in Patent Literature 4, a cylinder liner made of hypereutectic aluminum-silicon alloy and a cylinder block made of aluminum alloy which was cast so as to envelope the cylinder liner are disclosed.
On the other hand, as to a piston ring sliding on an inner circumference surface of a cylinder liner, in order to achieve the object of the reduction in weight of the engine, a further development has been also undertaken besides above-mentioned various kinds of cylinder liners.
For example, in Patent Literature 5, a piston ring for combining with a cylinder liner disclosed in the Patent Literature 4 is disclosed.    [Patent Literature 1] Japanese Patent Unexamined Patent Publication No. S63-038567 (JPS63-038567A)    [Patent Literature 2] Japanese Patent Unexamined Patent Publication No. H03-267549 (JPH03-267549A)    [Patent Literature 3] Japanese Patent Unexamined Patent Publication No. H04-076251 (JPH04-076251A)    [Patent Literature 4] Japanese Patent Unexamined Patent Publication No. H09-019757 (JPH09-019757A)    [Patent Literature 5] Japanese Patent Unexamined Patent Publication No. H11-287326 (JPH11-287326A)
However, sufficient performance cannot be obtained from the combination of the cylinder liner and the piston ring disclosed in Patent Literature 5, and thus there is still room for further improvement.
Specifically, for example, in recent years there has been a movement to reduce the frictional force between a cylinder liner and a piston ring for the purpose of improving fuel economy, and it is considered to reduce a tension rate (N/m) obtained by dividing the total tension of the piston rings by a bore diameter. However, the combination of the cylinder liner and the piston ring disclosed in Patent Literature 5 does not consider this point and it is far from obtaining the sufficient performance.
In addition, as for the cylinder liner disclosed in Patent Literature 4, a sufficient performance is not obtained. For example, as for such cylinder liner, there was a problem that the frictional force with a piston ring increases when a difference in level between silicon primary crystal or intermetallic compound such as Al2Cu or Mg2Si and Al alloy as a base material, and a corner edge of these compounds which extends to the sliding surface base side, which are caused by chemical polishing (etching), slides with the piston ring. In addition, when the tension of the piston ring is relatively large as in an oil ring (about 10 times as much as a pressure ring), the wear resistance of the ring and the aggressiveness to the cylinder liner sometimes became a problem.
Furthermore, not limited to the above-mentioned Patent Literatures 1 to 5, there have been many cases where a cylinder liner and a piston ring are developed separately, and it has not been considered to settle, with a total power of the cylinder liner and the piston ring, low friction properties with the cylinder liner and the piston ring as well as the reduction in weight of a whole engine and also the reduction of each other's amount of wear.